1. Field of Invention
An electrode having a valve metal substrate and an electrocatalytic surface composition comprising titanium dioxide doped with bismuth is provided, and an electrolytic water purification process utilizing this electrode, wherein organic substances dissolved or dispersed in water are oxidized and degraded in a nonselective manner with good current efficiency.
2. Prior Art
Electrodes with an electrocatalytic coating of doped diamond are known in the art and recommended for the same application; for example, U.S. Pat. Nos. 6,306,270 and 6,553,916. Production of these electrodes involves chemical vapor deposition or similar processes typically used in the production of integrated circuits; therefore, diamond coated electrodes are expensive to produce and limited to sizes compatible with semiconductor fabrication equipment.
Weres and Hoffmann, U.S. Pat. No. 5,419,824 provided an electrode (anode) for electrolytic water purification comprising a titanium metal substrate with an electrolytic coating comprising titanium dioxide (TiO2) doped with either niobium in the +4 valence state or tantalum in the +4 valence state applied directly to the Ti metal substrate. Weres and O'Donnell, U.S. Pat. No. 6,548,405 provided an electrode wherein an antipassivation coat (“precoat”) comprising oxides of iridium and tantalum was applied to the Ti-metal substrate, followed by an intermediate “seal coat” comprising tin dioxide doped with antimony, and finally an outer electrocatalytic coating comprising sintered particles of titanium dioxide doped with niobium in the +4 valence state cemented by a matrix of titanium dioxide doped with antimony. This structure provided good current yield for oxidation of organic compounds, together with long service life.
Kötz, U.S. Pat. No. 4,839,007 provided an electrode comprising a Ti metal substrate with an electrocatalytic coating comprising SnO2 doped with F, Cl, Sb, Mo, W, Nb or Ta (most preferably Sb) applied directly to the Ti metal substrate. Dietrich, U.S. Pat. No. 5,364,509 described an electrode comprising a Ti metal substrate with an antipassivation coating containing iridium, and an outermost electrocatalytic coating comprising SnO2 doped with Sb.
Many electrodes are known comprising a Ti metal substrate with an electrocatalytic coating containing one or more platinum group metals in metallic or oxide form. In some cases, Bi is specified or suggested as an additional constituent in combination with platinum group metals. Bismuth is known to form very stable, electrically conductive mixed oxides of the pyrochlore structure with some of the platinum group metals. Welch, U.S. Pat. No. 3,801,490 provides an electrode with an electrocatalytic coating comprising grains of Bi2Ru2O7 or Bi2Rh2O7 cemented with a glassy or cryptocrystalline matrix of another metal oxide, preferably TiO2. Welch teaches that the titanium dioxide serves as an unreactive cement that strengthens the coating, and that, while an electrocatalytic coating consisting entirely of the precious metal pyrochlore would provide an operative electrode, preferably the precious metal pyrochlore should comprise 10 to 80 weight percent of the electrocatalytic coating, with the balance TiO2. While primarily recommending this electrode for chlorine generation or metal recovery applications, Welch also teaches that it may be used to oxidize organic compounds, identifying as an example the product specific, partial oxidation of propylene to propylene oxide or propylene glycol. Welch's examples illustrate methods of producing these electrodes which ensure that the (Ru,Rh)—Bi pyrochlore and the titanium dioxide phase form distinct phases with essentially no mixing of Ti with the other metals. Nidola, U.S. Pat. No. 6,210,550 provided an electrode comprising a titanium metal substrate with an electrocatalytic coating essentially comprising IrO2 and Bi2O3, optionally also containing SnO2 and other metal oxides.
Koziol, U.S. Pat. No. 4,086,157 provided an electrode comprising a Ti-metal substrate, a base layer chosen to prevent passivation of the substrate, and an electrocatalytic layer believed to comprise at least two distinct phases: (1) a stable, electrically conductive metal oxide phase having spinel or similar crystal structure, and (2) a cementing matrix comprising titanium dioxide and/or tantalum pentoxide doped with up to 28 mol percent Nb, W, Mo, Sb or Sn to make the matrix electrically conductive. To ensure a sufficient content of the spinel or similar metal oxide phase, the aggregate concentration in the electrocatalytic coating as a whole of the oxides of the elements Ba, Ga, Ge, Pb, Bi, Se, Te, Cu, Cd, the rare earth elements, Mn, Fe, Co and Ni was specified to be greater than about 50 mole percent. No example including Bi in the coating was provided, nor is there any suggestion that combining the oxides of Bi and Ti alone would give an operative electrode. It is known in science and the art that only a few metals (notably Mn, Fe, Co and Ni) form spinels with no other metal present, and Bi that doesn't enter spinel phases due to the much larger radius of the Bi+3 cation.
De Nora, U.S. Pat. No. 4,272,354 provided an electrode comprising a Ti substrate coated with a solid solution of tin and bismuth oxides, preferably in the mole ratio 9:1 to 4:1 by weight of the respective metals. This mixture of Sn and Bi oxides can serve as the electrocatalytic layer itself, or else as an intermediate layer with an electrocatalytic layer containing platinum group metals applied over it. Also, De Nora teaches that the oxides of Sn and Bi may be added to an electrocatalytic coating comprising mostly the oxides of Ru, Ti and Co; examples are provided wherein the relative metal weight fractions of Ru:Ti:Sn:Bi:Co are 45:(35-55):(1-16):(0-5):(0-6). De Nora neither teaches nor suggests the combination of Ti and Bi in the absence of Ru or another platinum group metal in an amount sufficient to favor generation of chlorine.